An electrically programmable read-only memory (EPROM) can be electrically programmed to permanently store data. EPROMs rely on a charge tunneling phenomenon to permanently store a charge in an electrical conductor of each cell. Avalanche and Fowler-Nordheim effects are examples of the tunneling phenomenon. Essentially, an EPROM is programmed by applying certain voltages thereto, sufficient to draw electron charges through a thin insulator and become trapped in a conductive material, such as polycrystalline silicon or silicon nitride. The voltages applied to the cell for reading the contents thereof are insufficient to release the trapped charges, and thus, once electrically programmed, the cell retains the information for a long period of time.
In semiconductor memory technology, there is a constant effort to construct devices with higher circuit densities to accommodate more cells per unit of wafer area. As technology increases and device areas become smaller, the area required to isolate cells from each other makes up a larger percentage of the total area required by an EPROM. Thus, there is a need to reduce the area required to isolate EPROM cells from each other.